The root of the problem seems to be that related field's new_data dict is not a MultiValueDict, but a more-or-less usual dict (probably originating from a DotExpandedDict.) This causes all sorts of b0rkage when editing inline related models. A patch that fixes IndexError when the field in question is blank and makes strings as they were passed and not only first letter, though it's probably not correct. The problem exists on the dev server, haven't tried mod_python or anything else.